You don't really go into what you mean by 10% - 20% vs 100% of page impressions. If you mean you only have one ad per page in all those cases, this may mean that you have exhausted all of the ad revenue from your niche advertisers. If you have more than one ad per page and you removed all but one from the page, this could be quite natural as well as the top paying ones are supposedly always in the first ad located on the page.
On a related topic, I've recently been in a (I think) relatively rare situation to compare Adsense revenue on search engine traffic from G vs. that from B/Y and have noticed a major difference in PPC on ads served to visitors who arrived via B/Y vs G traffic. Could it be possible that B/Y people click on cheaper ads, or is G detecting where they are coming from and "smart charging" advertisers less for visitors coming from B/Y?
P.S. I'm with you, I still have my digital photo frame, although the wireless accessories pack broke soon after arrival.